Generally, engineering machines are characterized by large transmit power, slow movement, wide speed range and complex control process, which, are the very advantages possessed by hydraulic transmission. As a result, hydraulic drive systems are widely used in the field of engineering machinery. Moreover, many full-hydraulic engineering machines have been developed, e.g., full-hydraulic excavators, full-hydraulic bulldozers, full-hydraulic cranes, full-hydraulic road graders, full-hydraulic road rollers, full-hydraulic spreading machines, and full-hydraulic forklift trucks.
A hydraulic system generally includes a hydraulic pump, a hydraulic valve and a hydraulic actuator. The hydraulic pump converts mechanical energy of a prime mover into hydraulic energy of a hydraulic fluid. The hydraulic valve adjusts the pressure, flow rate, and direction of the hydraulic fluid. The hydraulic actuator converts the hydraulic energy of the hydraulic fluid into mechanical energy, performs a corresponding action and completes a predetermined operation.
Due to the diversity of operating environments and demands, engineering machinery requires hydraulic systems to have predetermined control functions, e.g., constant power control function, pressure shut-off function, load-sensing function, self power control function, cross power control function, negative flow control function, and positive flow control function. According to their differences in basic control principles, control functions of hydraulic systems can be classified into: speed control functions, power control functions and energy-saving control functions.
In a hydraulic system, the speed at which the hydraulic actuator operates depends on the pressure that the hydraulic fluid gives and the output flow rate of the hydraulic pump, the output power of the hydraulic system is also related to the pressure in the hydraulic system and the output flow rate of the hydraulic pump. Because the pressure in the hydraulic system is determined by the load, the control of the speed at which the hydraulic actuator operates and the control of the output power of the hydraulic system are actually realized by controlling the output flow rate of the hydraulic pump. The basic idea of energy-saving control is to balance the supply and demand of flow rate, i.e., to adjust the output flow rate of the hydraulic pump so that the flow rate of the hydraulic fluid required by the hydraulic actuator is correctly met, thereby reducing useless output hydraulic energy and achieving energy saving in the hydraulic system. Therefore, energy saving control is also realized by controlling the output flow rate of the hydraulic pump. As can be seen, control functions of hydraulic systems depend on the control of the output flow rate of the hydraulic pump.
The output flow rate of a hydraulic pump is related to the pump shaft speed and the displacement. The pump shaft speed is provided by a prime mover. In the industry of engineering machinery, engines are widely used as the energy source. In order to extend the engine's service life and to reduce its fuel consumption, speed control of the diesel engine is generally used, i.e., to maintain the suction power of the engine substantially constant so that the speed of the engine remains substantially constant, thereby avoiding the engine being affected by load surge in the hydraulic system. Hence, in practice, the pump shaft speed of the hydraulic pump is maintained substantially constant. Therefore, the control of the output flow rate of a hydraulic pump is actually the control of its displacement.
To realize automatic and adaptive adjustment of the displacement of a hydraulic pump, normally a displacement control mechanism is used. The displacement control mechanism adjusts the displacement of the hydraulic pump according to pressure changes at the outlet of the hydraulic pump, to meet a predetermined requirement. The basic principle of the displacement control mechanism adjusting the displacement of the hydraulic pump is: the displacement control mechanism receives a signal representing the outlet pressure of the hydraulic pump, and drives a variable displacement mechanism of the hydraulic pump to perform a predetermined action according to the outlet pressure of the hydraulic pump, thereby realizing adjustment of the displacement of the hydraulic pump. Specific control functions of hydraulic systems may be different, but the basic control principles behind them are generally the same, except for the specific transfer function between the variable displacement mechanism and the outlet pressure of the hydraulic pump. The operating principle of the displacement control mechanism is described below, along with a constant power control function of a hydraulic system as an example.
In a hydraulic system with a constant power control function, the displacement control mechanism has an input connected to an outlet of a hydraulic pump, and an output connected to a variable displacement mechanism of the hydraulic pump. The variable displacement mechanism normally includes a variable displacement piston. According to pressure changes at the outlet of the hydraulic pump, the displacement control mechanism drives the variable displacement piston of the hydraulic pump to perform a predetermined action via a mechanical structure and a hydraulic circuit, e.g., an up stroke or a down stroke, causing an appropriate change in the swash-plate angle of the hydraulic pump, changing the displacement of the hydraulic pump, thereby realizing adjustment of the output flow rate of the hydraulic pump. When the outlet pressure of the hydraulic pump increases, the displacement of the hydraulic pump is reduced, so as to lower the output flow rate of the hydraulic pump; when the outlet pressure of the hydraulic pump decreases, the displacement of the hydraulic pump is increased, so as to raise the output flow rate of the hydraulic pump, thereby maintaining the output power of the hydraulic pump substantially constant, making the hydraulic energy output by the hydraulic system at a substantially constant rate, and realizing constant power control of the hydraulic system.
As can be seen, the performance of a control function of a hydraulic system mainly depends on the performance of the control of the hydraulic pump, which in turn depends on the performance of the displacement control mechanism. Accordingly, obtaining characteristic parameters of the displacement control mechanism, to find out the performance of the displacement control mechanism, is a key in realizing a specific control function of a hydraulic system.
For a hydraulic pump with a constant power control function, the performance of its displacement control mechanism can be evaluated by a curve describing the relationship between the output power of the hydraulic pump and the pressure in the hydraulic pump. If, as the pressure varies, the output power of the hydraulic pump remains substantially unchanged, then the performance of the displacement control mechanism is considered good; otherwise, the performance is considered bad.
The output power of a hydraulic pump is related to the outlet pressure and the output flow rate of the hydraulic pump. In order to evaluate the performance of the displacement control mechanism for the hydraulic pump, a pressure parameter and an output flow rate parameter have to be obtained. Similarly, in a hydraulic system with a speed control function and an energy-saving control function, the evaluation of characteristics of the displacement control mechanism for the hydraulic pump should also be based on a pressure parameter and an output flow rate parameter.
The outlet pressure parameter of a hydraulic pump can be measured by a pressure measuring device, and the output flow rate parameter of a hydraulic pump can be measured by a flow meter. Alternatively, we can measure the pump shaft speed of the hydraulic pump and the swash-plate angle of the hydraulic pump, and obtain the output flow rate parameter according to the relationship between the pump shaft speed, the swash-plate angle and the output flow rate of the hydraulic pump.
Currently, the precision, real-time performance, and cost of pressure measuring devices can meet the measuring requirements. However, the measurement of the output flow rate of the hydraulic pump is not satisfactory. Flow rate measurement by flow meters has a poor real-time performance and a long response time, normally tens or even hundreds of times longer than the response time of a pressure measuring device, which degrades the reliability of the obtained output flow rate parameter. Moreover, control precision of flow meters is far from satisfactory in measuring the displacement control mechanism, with a measurement error many times larger than pressure measuring devices. Therefore, the measurement of the output flow rate of a hydraulic pump by a flow meter is far from satisfactory in evaluating the characteristics of the displacement control mechanism. In addition, flow meters cost far more than pressure measuring devices, i.e., the cost of a flow meter is normally a dozens times more than a pressure sensor. If we obtain the output flow rate of a hydraulic pump by measuring the swash-plate angle of the hydraulic pump, a swash-plate angle sensor that meets the measuring precision requirement will cost tens of times more than a pressure measuring device.
Therefore, currently the output flow rate parameter can not be obtained with high precision and high reliability at a low cost; moreover, the precision and reliability of evaluation result of the performance of a displacement control mechanism based on the output flow rate parameter can not be ensured at a low cost.